The present invention relates to a process of producing ultra thin and short metal fibers.
The metal fiber has variously excellent properties over a wide using range in comparison with non metallic fibers such as glass, carbon, and others, for example, high elasticity, flexibility, anti-abrasion, heat resistance, preferable conductivity of heat and electricity, or good wetted property.
The metal fibers are classified into a long fiber and a short fiber. The former is mainly used for grinding such as in a scrubbing pot cleaner or brush, and little used for the other usages. The latter is suitable for use in many kinds of composite materials such as mechanical frictional materials as a clutch or braking pad, strengthened plastic material, electrially conductive plastic material electrially conductive rubber material, electrification preventing material, or sealing material. Further, the short metal fiber is very useful for sintered porous substances such as a filter, heat pipe wig or heat exchanger, or as a metallurgical raw material, e.g., sound absorbing material, sound arresting material made by impregnating or mixing different substances in the porosities of the sintered material, or as base materials for the heat collector of the hot well or the chemical reaction catalyst.
Thus, short metal fibers are used widely as mentioned above. These are required to be less than 200 .mu.m in diameter and less than 20 mm in length, and in addition should have the physical properties of enough tensile strength. In these circumstances, the production of such fibers is more difficult than production of non metallic fibers. Techniques of more production of such fibers has not heretofore been realized to permit mass production of short metal fibers having industrially economical excellent properties.
Nowadays, three processes have been known as manufacturing the short metal fibers, that is,
I. Drawing wire and cutting process, PA1 II. Extraction from the molten metal and PA1 III. Making whiskers
With respect to the making of whiskers (III), a process is known which ulitize metallic hair crystalline, and may produce such ultra thin and short fibers such as several microns in diameter and less than 5 mm in length. However, it is difficult to obtain homogeneity of the fibers and the production cost is more expensive than the other two processes. Further, scattering is inferior when used to the composite material. Although this process is available on a laboratory scale, it has a problem in the industrial practice.
The process of extraction from molten metal (II) makes use of a cooled disc dipped partially in the molten metal for instantaneously providing cooling and solidification which spouts or scatters the molten metal from the cooled nozzle or disc. However, due to a restriction on thickness of the product, it is not practicable to make the fibers less than 200 microns in diameter, or provide ones of excellent strength.
For those actual problems, the drawing-cutting process (I) has broadly been employed. This process comprises rolling the cast ingot and producing the wire material, repeatedly passing the wire through many dies of various hole diameters, and finally subjecting the wire to a post-process of the cutter mill. This is, as seen, inferior in the production and very high in the production cost. Since the product is circular in cross section and has a smooth surface, the adherence is not preferable when compounded with other materials. Additionally, the porosity is not satisfied when the fibers are gathered.
The present inventors proposed a cutting process of manufacturing the short metal fibers, in which a plain milling cutter directly applies to a flat work metal block, and the production cost is low because of the work material of low process extent. However, since the rotating cutting edge is urged to the material while the milling cutter and the material are moved relatively for carrying out multi-cutting, the producing conditions are severe, and the size or shape of the fiber or the physical properties are changed by disorderings such as vibration of a main shaft of the machine, clearance between an arbor and an inner diameter of the cutter, or minute error on precision as vibration of an outer circumference of the cutting edge. Although it is possible to manufacture thick fibers up to 0.5 to 0.8 mm in diameter and 25 to 50 mm in length for reinforcing the structural concrete, the ultra thin and short fibers cannot be produced as less than 200 microms in diameter, e.g., 5 microns to the minimum even if the machine or tool are finished with the super precision and imparted with the high rigidity, and the obtained fibers are broad in dispersion in the various characteristics. Further, there are also problems in the production due to the on-and-off processing by the tool having the equidistant cutting edges on the outer circumference thereof. In these circumstances, satisfied ultra thin and short fibers have never been produced in mass at the low cost in dependence upon this process.